In the solid phase, molecules are bound closely to one another in an orderly fashion. A solid holds its shape. The volume of a solid is fixed and bound by the shape of the solid. When energy is added to a solid, the structure of the molecules loosens and becomes liquid. There are two kinds of solids, crystalline solids such as quartz and amorphous solids like gels.
In the liquid phase, molecules are held together less tightly by molecular forces. A liquid takes the shape of the container in which it is held. Liquid has a fixed volume, regardless of the shape of its container. When a liquid is heated, it vaporizes into gas. Water is the most frequently occurring liquid.
In gaseous phase, molecules are not well held together. They are widely spread, and move around freely. Gas takes the shape and volume of its container. Gas is characterized by constant, random motion of molecules, perpetually colliding against one another and the container. Oxygen, nitrogen, and hydrogen are gases that occur naturally and prevalently at room temperature.
If enough heat is added to a gas, the protons and electrons separate. This state is known as plasma. Like gas, plasma has no definite shape or volume. An attribution exclusive to plasma, however, is its ability to react to electromagnetic fields. Plasma lamps and stars are an everyday example of plasma.